What weight to use is based on expected outside temp. I have seen some owner's manuals saying not to use 5W-30 if it's 60 F or higher and not to use it on long highway trips during the summer months. I don't remember what car it was for, but this just goes to show it is based on the car and climate. For my car, the owner's manual say a good year-round oil would be 10W-40 or 50. It's good from -4F to 86F. Currently I have 15W-40 in it, which per the graph in the manual, is good from 32F-86F.
Now as far as the old syn. vs. dino. question, like Jim said there is currently thread on this subject. Among gasoline owners, Mobil 1 seem to be the most popular for synthetic oil. For diesel owners using synthetic, it's either Mobil 1, or Mobil 1 Delvac.
1994 MB E320 Coupe (199,500 mi)
1992 MB 190E 2.6 (173,000 mi)
1983 Porsche 911SC (148,000 mi)
2004 Volvo V70 2.5T (198,000 mi)
President of CT/Westchester Section